1. Field of the Invention
The present invention relates to an ultrasonic motor for use in such fields as controlling a microscope stage, in which positioning steps are required.
2. Description of the Related Art
In recent years, ultrasonic motors that realize high-speed operations and inching operations over a relatively wide range while being able to be configured to be small in size have gathered interest as actuators for precision instruments. One of the ultrasonic motor types is the ultrasonic motor that is configured as disclosed in Patent Document 1 (Japanese Patent No. 2722211), Patent Document 2 (Japanese Patent Application Publication No. 2004-201380), and Patent Document 3 (Japanese Patent Application Publication No. 2005-295656). The ultrasonic motor disclosed in these documents employs a configuration in which the longitudinal vibrations (also referred to as the stretching vibrations) and the bending vibrations are excited at the same time, and thereby elliptical vibrations are caused at the position at which the ultrasonic motor contacts the driven object in order to move the driven object in desired directions. A piezoelectric body is commonly employed as a material for the ultrasonic motors. By applying alternating currents having different phases to a longitudinal vibration electrode and a bending vibration electrode formed on the piezoelectric body, the elliptical vibrations are caused. Because of the application of the alternating currents having different phases, when the deformation direction of the longitudinal-vibration-electrode portion in the piezoelectric body and that of the bending-vibration-electrode portion in the piezoelectric body overlap each other, one electrode portion in the piezoelectric body may be forced to shorten while the other electrode portion stretches.
Also, the ultrasonic motors are configured to achieve a large vibration amplitude by utilizing their own resonance frequency. Accordingly, when the resonance frequency that was set as the initial setting is changed due to a change in the ambient temperature or in the contacting state between the motor and the driven object, the vibration amplitude becomes smaller and the moving distance of the driven object is affected. In order to cope with this problem, an ultrasonic motor as disclosed in FIG. 4 of Patent Document 4 (Pamphlet of WO 04/088830) is suggested in which a vibration detection electrode is provided for monitoring the vibrating state of the vibrator.
Also, when an ultrasonic motor is to be used in a state in which it is incorporated with a microscope stage or the like, pre-load has to be applied to the vibrator to some extent in order to prevent the movable portions of the stage from being moved easily (for example, a pre-load of about 20 N is required in order to achieve a holding force of about 5 N). This condition requires the ultrasonic motor to be able to output power that is sufficiently high, and also to be rigid, which is achieved by using materials that are thick enough and wide enough to sustain such a pre-load. In order to meet this requirement, a larger current has to flow to the driving electrodes, especially if the total amount of the current flowing to the GND becomes large because the current that has flowed to the respective driving electrodes flows to the GND. In the case of, for example, the vibrator disclosed in Patent Document 5 (Japanese Patent Application Publication No. 2005-65358), there is only one GND electrode while there are four driving electrodes, and there is only one connecting point for the lead wire. Accordingly, in order to prevent the lead wire of the GND from being disconnected due to heat or the like, a lead wire having a larger diameter is required for allowing larger current to flow.